1. Field of the Invention
This invention relates to methods for preparing ZSM-5 zeolite materials.
2. Description of the Prior Art
Zeolites are porous materials having tridimensional networks of micropores. This class of material is key to various industrial processes involving catalysis, ion-exchange and adsorption/separation (molecular sieving) processes. The framework of a normal zeolite contains tetrahedral Si and Al atoms. The Al sites are particularly important for ion exchange processes. This invention is more particularly concerned on the class of zeolite materials known as ZSM-5.
Various methods of preparing or conditioning ZSM-5 zeolites are known in the art. For instance, it is known that controlled desilication (removal of Si tetrahedral sites) of ZSM-5 zeolites by mild caustic leaching decreases the Si/Al ratio (up to 25%) without provoking significant structural collapses or losses of Al tetrahedral sites [1-6]*. Controlling the extent of desilication is achieved by carefully controlling temperature, time and initial pH of the desilicating solution. As general rule, the higher the Si/Al atom ratio of the parent ZSM-5 zeolite, the lower must be the initial pH of the desilicating solution [3].
 FNT *Numbers indicated in square brackets refer to art references provided as an appendix to this disclosure.
It is interesting to note that during the desilication operation and during the subsequent water washing of the desilicated zeolite materials, sodium orthosilicate and its dimer, sodium pyrosilicate, are selectively removed from the zeolite framework [5].
As a result of desilication, the density of tetrahedral Al sites is increased. This was reported to improve ion-exchange properties [2,5,6], and catalytic behavior during catalytic dehydration of ethanol [7].
It is also known that thermal treatment [5] or steaming [4] of the desilicated ZSM-5 zeolite further enhances the ion-exchange and the catalytic performances of the materials. Desilicated ZSM-5 when simply dried at 150.degree. C. exhibit new and useful micropores, much narrower than the original ones (about 0.49 nm instead of 0.52-0.54 nm for the parent zeolite) [5].
However, these gains are rapidly erased when the above described zeolite materials are subjected to higher temperatures or steam treatments. Indeed, such treatments render the zeolite material unstable at temperatures lower than 350.degree. C. and causes the appearance of significantly larger pore sizes at temperatures higher than 350.degree. C. All micropores, including the new small micropores (0.49 nm) revert to significantly enlarged micropores (0.55-0.56 nm) upon treatments at high temperatures, in the presence of steam [4] or simply by heating in the air above 350.degree. C. [5]. Such a dependance of micropore size on temperature prevents any commercial application at high temperatures.
It is therefore of utmost importance to develop methods which allow desilicated ZSM-5 zeolite material to achieve high stability and retain their micropore sizes under normal industrial conditions such as the ones found in catalytic cracking processes (high reaction temperatures and frequent regeneration).
In addition, obtaining a stable desilicated ZSM-5 zeolite with a pre-selected micropore size distribution depending on the intended industrial application is a highly coveted goal. Conventional ZSM-5 zeolite materials have a pore size diameter of about 0.52 to 0.54 nm. Obtaining a stable array of smaller diameter pores (about 0.49 nm) or larger diameter pores (about 0.55 or larger) would be quite useful. Indeed with a stable micropore size distribution of smaller or larger micropore sizes, the resulting zeolite could be used as a catalyst capable of expressing shape-selectivity (or molecular sieving effect). This feature being of course different or less pronounced for the parent ZSM-5 zeolite.
Thus, it is the main object of this invention to overcome the drawbacks of the prior art by providing a method for the stabilization of desilicated ZSM-5 zeolites and for selecting micropore size distribution.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that this detailed description, while indicating preferred embodiments of the invention, is given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art.